Most people think of welding as requiring a torch or arc that is hot enough to melt the materials being welded. However, a known conventional kind of welding called friction stir welding (FSW) is a solid-state joining process that can join materials without melting them. It is commonly used for applications where it is helpful that the original material characteristics remain largely unchanged. Friction stir welding can be used to weld aluminum, magnesium, copper, titanium, steel, and some plastics.
To accomplish friction stir welding, a specially designed rotating tool heats up and mixes the interface portions where two parts meet. This heating and mixing of the materials in solid state joins the parts without causing them to melt. The rotating tool is typically in the shape of a pin mounted on a rotating spindle. The tool has a shoulder that doesn't penetrate into the material to be welded, but rotates over it. This rotation generates friction and consequently thermal energy that softens the material to be welded. The stirring then joins the two parts on a molecular level so the two parts essentially become one.
FSW provides a number of potential advantages over conventional fusion-welding processes such as for example:                Good mechanical properties of the welded workpiece without need to melt the workpieces;        Improved safety due to the absence of toxic fumes or the spatter of molten material;        Welding patterns are easily automated on relatively simple milling machines;        Can operate in all positions (horizontal, vertical, etc);        Generally good weld appearance and minimal thickness under/over-matching, thus reducing the need for expensive machining after welding;        Low environmental impact;        Other.        
During friction stir welding, a number of forces will act on the rotating tool. For example, a downwards force is used to maintain the position of the tool at or below the material surface. A traversal force acts parallel to the tool's motion. A lateral force may act perpendicular to the tool traverse direction. A torque is used to rotate the tool. How much torque is used will depend on the downforce and the friction coefficient (sliding friction) and/or the flow strength of the material in the surrounding region (sticking friction).
In many cases, the vertical position of the tool is preset and so the load will vary during welding. However, friction stir welding machines that automatically control some or all of these various forces to provide constant downforce have certain advantages. For example, example friction stir welding equipment may include actuators and sensors that are able to automatically control the position, orientation and motion of the tool. Some example friction stir welding systems include various sensors such as load cells, pressure sensors and displacement sensors that sense the position of the tool and the amount of force the tool is applying. A control system can be used to control tool position and downforce in response to these sensed parameters.
In order to prevent tool fracture and to minimize excessive tool wear, it is generally desirable to control the welding operation so that the forces acting on the tool are as low as possible and sudden changes are avoided. Conditions that favor low forces (e.g. high heat input, low travel speeds) may however be undesirable from the point of view of productivity and weld properties. While constant downforce is a desirable design goal, because of the many factors involved it can be difficult to achieve. Complete safety from the tool colliding with the backing surface is often not possible due to slight warpage or other distance variations of the backing relative to the tool. Thus, further improvements are possible and desirable.
Certain example non-limiting technology herein provides friction stir welding equipment and methods, developed according to requirements of high reliability, robustness, precision and low cost, in order e.g., to weld lap and butt joints in complex surfaces with fixed or substantially constant pin tool control force.
Exemplary illustrative non-limiting equipment comprises a control force spindle mounted in an orbital head housing. A coaxial sensor measures downforce. Simultaneously, an axial electrical actuator is controlled to dynamically correct the axial tool position during the welding, by a direct axial force system control, in order to maintain controlled downforce according to parameters previously set, based on numerical control. The equipment also sets up, monitors and controls spindle rotation speed, welding speed, acceleration speed and downforce using for example closed loop control functions. The exemplary illustrative non-limiting implementation may also record in a database the downforce and tool welding position during welding.
In addition, exemplary illustrative non-limiting equipment comprises a laser system that scans the backing surface before welding and corrects original tool path, in order to get an offset tool path. A precision alarm system provides safe welding while preventing the tool from colliding with the backing surface.
An example non-limiting method of performing friction stirred welding comprises:
(a) measuring the downforce that a rotating friction stirred welding tool applies to a workpiece; and (b) controlling an electrically controlled actuator based on numeric control while correcting axial tool position at least in part in response to said measured downforce to thereby maintain the load between tolerance limits, said controlling including avoiding oscillations of the load applied to the workpiece by applying proportional integral derivative control to maintain the load constant or substantially constant during welding.
The method can further include measuring variations in axial distance between the tool and the workpiece. The method can further include measuring variations in axial distance between a spindle into which the tool is mounted and a backing onto which the workpiece is placed, and using said measured variations to correct axial tool position and avoid collision between said tool and the backing. The method can further include generating an alarm if the axial distance between the tool and the backing is less than a predetermined threshold distance determined based at least in part on said measured variations. The method can further include logging welding parameters during welding. The method can further include controlling rate of rotation of said tool using a closed loop control process.
The exemplary illustrative technology herein further provides a friction stirred welding system of the type including a spindle mounted in a orbital head housing, said spindle having a rotating tool mounted therein, said tool rotating in contact with a workpiece, the axial position of said tool being determined by an electrically controlled actuator. The system may comprise a sensor that measures the downforce the rotating tool applies to said workpiece. The system may further comprise a control system coupled to said sensor, said control system being structured to control said electrically controlled actuator to correct axial tool position at least in part in response to said measured downforce to thereby maintain the load between tolerance limits, said control system being further structured to avoid oscillations of the load applied to the workpiece by applying proportional integral derivative control to maintain the load constant or substantially constant during welding.
The system may further include a laser sensor that is adapted to be accepted by the spindle and interchangeable with said the rotating tool, said laser sensor measuring variations in axial distance between the rotating tool and the backing surface.
The system may further include a laser sensor that is adapted to be accepted by the spindle and interchangeable with the rotating tool, said laser sensor structured to measure variations in axial distance between the spindle into which the tool is mounted and a backing surface onto which the workpiece is placed, and said control system using said measured variations to correct axial tool position and avoid collision between said tool and the backing.
The system may further include an alarm that indicates if the axial distance between the tool and the backing is less than a predetermined threshold distance determined based at least in part on said measured variations.
The system may further include a data logger that logs welding parameters during welding.
The system may further include a closed loop control arrangement that controls rate of rotation of said tool.
The exemplary illustrative non-limiting technology herein further provides a method of performing friction stirred welding comprising: (a) inserting a sensor into a friction stirred welding spindle; (b) using the sensor to map the axial distance the friction stirred welding spindle is disposed from a backing surface; (c) removing said sensor from said spindle and inserting a tool in its place; (d) rotating said tool; (e) moving said rotating tool into contact with a workpiece placed on said backing surface; and (f) using said map to control an electrically controlled actuator to correct axial tool position relative to said workpiece, wherein said rotating tool in contact with said workpiece plasticizes portions of said workpiece while keeping said workpiece in the solid state, thereby welding said workpiece.
The method may further avoid oscillations of the load applied to the workpiece by applying proportional integral derivative control to maintain downforce of said tool constant or substantially constant during welding.
The method may measure variations in axial distance between a spindle into which the tool is mounted and a backing onto which the workpiece is placed, and using said measured variations to correct axial tool position and avoid collision between said tool and the backing.
The method may generate an alarm if the axial distance between the tool and the backing is less than a predetermined threshold distance determined based at least in part on said measured variations.
The method may automatically log welding parameters during welding.
The method may control rate of rotation of said tool using a closed loop control process.